1. Technical Field
The present invention relates to a device for measuring action force of a wheel for detecting road surface frictional force, vertical drag, road surface friction coefficient, etc. which can be a constituent element of a brake system with skid control for preventing lock (seizure) of the wheels of a vehicle on sudden stops or a traction control system for preventing excessive slip of the wheels at acceleration, and a device for measuring stress of a structure for measuring stresses produced in other structures.
2. Prior Art
The antilock brake system (ABS) of the conventional vehicle such as an automobile generally employs the principle of controlling the brake action automatically according to the chassis speed and wheel speed in such a manner that the slip ratio will fall with a certain range [cf. Japanese Patent Publication No. 59-30585 and Japanese Kokai Patent Publication No. 60-61354). The relationship between road surface friction coefficient and slip ratio is variable according to road surface condition. Therefore, in the above-mentioned system, the braking force may not be maximal depending on road surface condition and the minimum braking distance cannot be obtained in such cases. Furthermore, since the chassis speed is estimated from the wheel speed, control of the slip ratio is deficient in precision. Exact determination of the chassis speed requires a complicated apparatus such as a ground-relative speed sensor (cf. Japanese Kokai Patent Publication No. 63-64861), a chassis deceleration sensor (cf. Japanese Kokai Patent Publication No. 63-170157) or the like.
There was accordingly proposed an ABS which takes into account the road surface frictional force or road surface friction coefficient as a measured value. In the system described in Japanese Kokai Patent Publication No. 63-25169, the torque (tire torque) of the road surface frictional force acting on the wheels is calculated from the wheel angular acceleration and brake fluid pressure and the beginning of decline in tire torque during the incremental phase of brake fluid pressure is adopted as one of the criteria for assessment of the wheel condition immediately before lock. However, in this system, the tire torque is indirectly determined by arithmetic operation from the wheel angular acceleration and brake fluid pressure but because of indefinite constants such as the wheel inertia efficiency and the braking efficiency of the brake, the precision of calculated values is not sufficiently high.
Regarding the traction control system, too, the conventional apparatus detects the wheel slip at acceleration by measuring the wheel speed just as the ABS. In this case, too, there is the same problem as in the ABS wherein control is based on the wheel speed. There is a demand for an apparatus for measuring dynamic quantities relating to the interaction between the road surface and the wheel such as road surface frictional force, vertical drag and road surface friction coefficient as a constituent of the new ABS or traction control system.